This progress report reviews the most recently published data obtained from our studies of cytotoxicity mechanisms in Inflammatory Bowel Disease (IBD). We have found that the effector cell of the cytotoxicity for colonic cells, which is demonstrated in vitro by peripheral blood mononuclear cells from these patients, is a "null" or "K" cell. Also we have shown that the effector cell of the cytotoxicity for colonic cells which is induced in vitro by pretreatment with IBD serum of peripheral blood mononuclear cells from healthy volunteers, also is a "null" or "K" cell. These findings support our hypothesis that this cytotoxicity is a form of antibody or antigen-antibody complex dependent cell-mediated cytotoxicity. However, the time necessary for IBD serum to induce the cytotoxicity for colonic cells shown by pre-treated normal mononuclear cells indicates that the active factor(s) in IBD serum is unlikely to be acting simply by passive coating of the "K" cells. Consequently, now we are restudying these sera, and the supernatants of cultures of IBD mononuclear cells (which have similar properties), to define these factor(s). In addition, we have developed a new system for studying the cytotoxic properties of peripheral mononuclear cells from patients with Chronic Active Liver Disease (CALD), a disease which has a recognized association with IBD, and this progress report describes our findings, including our preliminary observations on the effector cell and the cytotoxicity-inducing properties of CALD sera.